1. Field of the Invention
The present invention relates to a rotating prism Q-switch for use with CO.sub.2 TEA (carbon dioxide transversely excited atmospheric) lasers. The rotating prism Q-switch angularly sweeps through alignment with the resonator mirrors once per revolution. An opto-electronic timing device with imaging optics rotating with the prism triggers the gas discharge at the proper time prior to resonator alignment.
2. Description of Related Art
The CO.sub.2 laser has long been available and can be configured to produce a continuous or pulsed laser beam. It is capable of high average power output while at the same time maintaining the high degree of spectral purity and spatial coherence, characteristics of the lower power atomic gas lasers. An electric discharge is the most common means of excitation. Operating efficiency and output power are greatly increased by adding nitrogen and helium to the fill gas. Helium aids depopulation of the terminal laser level and nitrogen excites the carbon dioxide molecules by collisional energy transfer. To facilitate the discharge, the CW (continuous-wave) excited CO.sub.2 laser is operated at low pressure, on the order of 100 torr.
Because of the long lifetime of the vibration levels, it is possible to store energy in the discharge medium by blocking the path of the laser beam within the resonator, thereby preventing the laser oscillation. If the block is suddenly removed, then the output from the laser occurs in the form of a sharp pulse with peak power two to three orders of magnitude larger than the average continuous-wave power obtainable from this laser. This mode of operation is called Q-switching. In a typical prior art device in which the gas is excited by CW discharge, Q-switching is accomplished by replacing one of the laser cavity mirrors with a rotating mirror. A laser pulse at 10.6 microns is produced every time the rotating mirror lines up with the opposite stationary mirror.
A more efficient method of producing high peak power pulses from the CO.sub.2 TEA laser is the use of a pulsed high voltage discharge in a gas medium at much higher pressure. As is known, a CO.sub.2 TEA (transversely excited atmospheric) laser is a type of CO.sub.2 laser in which excitation of the active medium is transverse to the laser beam axis and, because of a shorter breakdown length, can operate in a gas pressure range higher than that for longitudinally excited gas lasers, thus achieving a higher power output per unit volume because of the greater density of lasing molecules. In this laser the gas pressure is near one atmosphere and the discharge is very fast and transverse to the beam axis. By operating at higher pressure, the density of excited CO.sub.2 molecules is increased, thereby proportionally increasing the peak power output. The difficulty of creating the discharge in the higher pressure gas is offset by the reduced path length of the transverse discharge. The high peak power of the CO.sub.2 TEA laser is not accomplished by a Q-switch, but results from the fast discharge which causes the gain to build up faster than the laser pulse. This method is called "gain switching."
The fast discharge method is undesirable for many laser applications because sufficient nitrogen excitation remains after the initial laser pulse to sustain laser oscillation at a power level 1/10 to 1/4 of the peak. The output energy after the main pulse is referred to as the "tail" and typically contains more than half the energy and lasts up to several microseconds. In laser range finder applications, the tail is backscattered into the receiver, thus "blinding" the receiver for the few microseconds that the tail exists, which blinding is unacceptable.
The tail can be eliminated by the addition of a Q-switch wherein the Q-switch is on for the main pulse, and then turned off to prevent the tail. The Q-switch also can increase the peak output power by delaying the switch opening so that the laser pulse occurs near peak gain. For the gain switched laser, the pulse can occur well before peak gain, thus increasing the tail energy.
Since the CO.sub.2 TEA laser has an excited state lifetime of only a few microseconds, a timing accuracy of a few hundred nanoseconds is required for the time delay between the gas discharge and Q-switch opening. Heretofore, only the electro-optic Q-switch was capable of providing this degree of timing accuracy. However, the electro-optic Q-switch has serious disadvantages when used with CO.sub.2 TEA lasers due to its cost, complexity, fragility and susceptibility to laser damage.
What is desired is an arrangement wherein a Q-switch can be utilized with the CO.sub.2 TEA laser without the aforementioned disadvantages of the electro-optic Q-switch, the timing accuracy required for laser operation still being provided.